It is frequently desired to deposit a film of titanium nitride over a film of titanium to prevent chemical attack of titanium by fluoride gas liberated in a subsequent tungsten deposition step. The commonly-used methods of forming titanium and titanium nitride films are generally incompatible. The titanium can be sputter deposited or deposited by plasma-enhanced chemical vapor deposition wherein titanium nitride is generally formed by low pressure chemical vapor deposition. In the formation of integrated circuits, metal may be deposited over a metal oxide semiconductor structure to react with exposed silicon areas such as source and drain regions to form metal silicides. Following the formation of the silicide regions, a selective acid etch has been used to remove unreacted metals without attacking the silicide. After the CVD is completed, the substrate is removed from the reactor, allowed to cool to room temperature and etched with an acid such as a mixture of hydrogen peroxide and sulfuric acid. This etch will remove any metallic titanium on the substrate as well as any substoichiometric TiSix formed on the silicon dioxide. This method of forming suicides is disclosed in U.S. patent application Ser. No. 08/489,040 entitled METHOD FOR FORMING SALICIDES, filed Jun. 9, 1995 (inventor Arena) herein incorporated in its entirety by reference.
Sputter deposition requires the utilization of a separate reaction chamber. In applications where a first film is deposited by chemical vapor deposition, which is the preferred method, followed by sputter deposition of a second film, two different chambers are required. This could then be followed by a third chamber where, for example, a metal layer would be sputter deposited. It is preferable to minimize the transport of the substrate from one reaction chamber to another and to conduct as many reactions as possible in a single chamber.
While physical sputtering provides deposition of a titanium film at a lower temperature, sputtering processes have various drawbacks. Sputtering normally yields very poor step coverage. Step coverage is defined as the ratio of film thickness on the bottom of a contact on a substrate wafer to the film thickness on the sides of the contact or the top surface of the substrate. Consequently, to sputter deposit a predetermined amount of titanium at the bottom of a contact or via, a larger amount of the sputtered titanium must be deposited on the top surface of the substrate or the sides of the contact. For example, in order to deposit a 200 .ANG. film at the bottom of a contact using sputtering, a 600 .ANG. to 1000 .ANG. film layer may have to be deposited onto the top surface of the substrate or the sides of the contact. Since the excess titanium has to be etched away, sputtering is wasteful and costly when depositing layers containing titanium.
The step coverage of the contact with sputtering techniques decreases as the aspect ratio of the contact or via increases. The aspect ratio of a contact is defined as the ratio of contact depth to the width of the contact. Therefore, a thicker sputtered film must be deposited on the top or sides of a contact that is narrow and deep (high aspect ratio) in order to obtain a particular film thickness at the bottom of the contact than would be necessary with a shallow and wide contact (low aspect ratio). In otherwords, for smaller device dimensions in an IC, corresponding to high aspect ratio contacts and vias, sputtering is even more inefficient and wasteful. The decreased step coverage during sputter deposition over smaller devices results in an increased amount of titanium that must be deposited, thus increasing the amount of titanium applied and etched away, increasing the titanium deposition time, and increasing the etching time that is necessary to remove excess titanium. Accordingly, as IC device geometries continue to shrink and aspect ratios increase, deposition of titanium-containing layers by sputtering becomes very costly.
There are significant costs associated with each individual process that decreases the throughput of a IC chip processor. This includes the time to heat a wafer, stabilize the reaction chamber pressure and gas flows, and stabilize rotation. The acid etch step described above significantly reduces the manufacturing rate. By transferring the wafer from station to station, there is a time delay, in this case, between the deposition of the titanium and subsequent nitridation and deposition of the titanium nitride film. During this time, the titanium film will undergo oxidation which can degrade the electrical properties of the film.
Previously, titanium and titanium nitride have typically been deposited using physical vapor deposition (PVD) methods such as sputtering. Using PVD, thick films of Ti and TiN must be deposited on the top layers of the device in order to achieve adequate bottom coverage.